“Crude” organic pigments, i.e., pigment in the form initially obtained after being chemically synthesized, are often unsuitable for use as pigments and must be subjected to one or more additional finishing steps that modify their particle size, particle shape, and/or crystal structure to provide good pigmentary qualities. This is described, for example, in K. Merkle and H. Schafer, “Surface Treatment of Organic Pigments” in Pigment Handbook, Vol. III (New York: John Wiley & Sons, Inc., 1973), page 157; R. B. McKay, “The Development of Organic Pigments with Particular Reference to Physical Form and Consequent Behavior in Use” in Rev. Prog. Coloration, 10, 25-32 (1979); and R. B. McKay, “Control of the application performance of classical organic pigments” in JOCCA, 89-93 (1989).
The most commonly used conditioning methods involve dissolving or suspending the crude pigment in strong mineral acids, followed by precipitation, and/or milling the crude pigment. Conditioning with a strong acid involves treating the crude pigment with aqueous mineral acid such as sulfuric acid in a process known as “acid pasting” (in which an acidic solution containing protonated pigment is formed) or “acid swelling” (in which a suspension of protonated pigment is formed). After the acid treatment is completed, the pigment is precipitated by adding the strongly acidic solution to a liquid in which the pigments are completely or almost completely insoluble, such as water or methanol or other lower aliphatic alcohols, as well as mixtures thereof.
Indanthrone pigments are well known polycyclic aromatic pigments. They require conditioning before use in fibers and plastics. However, attempts to condition these pigments by salt grinding or acid pasting methods has led to products which had either poor strength and color or poor dispersability. For instance, a salt ground product was close to the desired strength and color but it did not have acceptable dispersability. Particle size measurements have shown that the size distribution of the final pigment was largely determined by the original crude size distribution and that a significant amount of undersized particles were present in the conditioned product. Presumably, these undersized particles produce tight aggregates which do not re-disperse under the intended conditions of use (extrusion processing to fibers and plastics).
Among the literature describing conditioning indanthrone pigments are:
DE 3522680 discloses crude isoindolines which were converted to pigmentary form by heating at high temperatures in alkyl benzoates after milling.
U.S. Pat. No. 6,013,126 discloses salt grinding organic pigments, including indanthrone (PB 60), then kneading with alkanoic acids to improve dispersability in plastics.
JP11269401 discloses surface treating organic pigments, including indanthrone (PB 60), resin acids onto the surface of the pigment in the presence of an anionic surfactant to give pigments with excellent dispersion and gloss properties.
JP11130974 discloses preparation of organic pigments, including indanthrone, by dissolving in an aprotic solvent (DMSO) in the presence of a base (NaOH) and water or a C1-C4 alcohol, and then quickly precipitating the pigment particles by addition to water, alcohol and/or an acid (H2SO4).
U.S. Pat. No. 5,900,050 discloses treating organic pigments, including indanthrone (PB 60), quaternary ammonium compounds or amines to give pigments with good dispersability in plastics.